1. Field of the Invention
The present invention generally relates to a pulling-in circuit for a PLL (Phase Locked Loop) circuit and an apparatus for reproducing an optical disc by use of the pulling-in circuit, and more particularly relates to the pulling-in circuit and the reproducing apparatus used in a case where a frequency to follow up (i.e. the clock frequency) is drastically changed, in a case of reproducing a CLV (Constant Linear Velocity) type disc by a CAV (Constant Angular Velocity) reproducing operation, for example.
2. Description of the Related Art
The CD for music is well known as an information record medium which has a memory capacity of large volume. Thus, there has been developed a CD-ROM, which is a CD recorded with the computer readable data such as program and data base instead of music signals. The CD-ROM is advantageous in that it is suitable for the mass production so that the production cost is relatively low. Thus, the CD-ROM has been a very popular record medium for the recent multi-media.
Since the CD-ROM is recorded with the data for computer or the compressed digital video signals etc. instead of the music signals, from the view point of keeping the high recording density, the CD-ROM is basically recorded as a CLV type record medium, i.e. recorded by the constant linear velocity, in the same manner as the CD for music. The CLV recording and reproducing method is a method in which the primary importance is given to the record density rather than the access time i.e. the time required for the optical pickup to move to a specific position where the desired data to be accessed are recorded on the optical disc.
In case that optical disc is used as an external ROM for the computer, it is preferable to decrease the access time. However, in case of the CD-ROM of CLV type, in order to keep the linear velocity constant during the rotation of the optical disc, it is necessary to change the rotation angular velocity in accordance with the radial position on the optical disk. Namely, the rotation angular velocity for reading a position near the inner circumference on the optical disc should be much higher than that near the outer circumference on the optical disc in the CLV type reproduction. Here, since the change in the rotation angular velocity of the disc requires a certain period of time, the CD disc of CLV type has an essential disadvantage to decrease the access time.
Therefore, there may be proposed to reproduce the CD disc of CLV type by the CAV type reproducing operation to reduce the access time. However, since data on the CD disc of CLV type are recorded on the assumption that they will be reproduced under the condition that the linear velocity is constant (i.e. by the CLV reproducing operation), if the CD disc of CLV type would be reproduced under the condition that the angular velocity is constant (i.e. by the CAV reproducing operation), the transmission rate of the data is greatly changed between the reproduction near the outer circumference of the disc and the reproduction near the inner circumference of the disc. In other words, the record density is apparently increased more as the optical pickup moves outward on the CLV disc during the CAV type reproducing operation, so that the transmission rate is increased more as the optical pickup moves outward.
More concretely, assuming that the number of rotation of the disc is about 1000 r.p.m. (rotation per minute), the data transmission rate (i.e. the rate of reading out the data) is about 350 kBytes/sec at the most inner circumference on the disc (which corresponds to about 2.8 MHz when converted into the clock frequency) while it is about 875 kBytes/sec at the most outer circumference on the disc (which corresponds to about 7 MHz when converted into the clock frequency). Namely, the difference in the transmission rate therebetween is as much as about 2.5 times.
In this case, in order to read the record information on the optical disc, it is necessary to change the frequency of an output clock signal for reading operation in accordance with the change in the data transmission rate (i.e. the clock frequency to follow up). The clock signal is generated by a PLL (Phase Locked Loop) circuit and is used for a servo control for the spindle motor which rotates the optical disc, tracking and focusing servo controls for the optical pickup which read the information on the optical disc, a carriage servo control for the carriage which carry the optical pickup, and so on. The PLL circuit generates the output clock signal such that the frequency and phase of the output clock signal is changed in accordance with the reading position on the optical disc. In order to achieve the oscillation in the certain frequency and phase range to follow up the data transmission rate (input clock signal) during the reproduction, the PLL circuit is provided with a pulling-in circuit to pull-in the oscillation within the sweep frequency range having a predetermined width into the frequency corresponding to the data transmission rate.
However, if the CLV type disc is reproduced by the CAV type reproducing operation, the PLL operation is difficult since the stable and speedy pulling-in operation for the PLL circuit is difficult due to the drastic change in the transmission rate along the radial direction on the CLV disc.
Namely, in the above mentioned PLL circuit, regardless of the reading position of the optical pickup, i.e. regardless of the radial position on the optical disc corresponding to the reading position, the sweep frequency range for pulling-in (corresponding to Vm to Vn) is fixed. That is to say, the upper and lower limit values of the sweep range are constant. Accordingly, for example, the sweep range suitable for performing the pulling-in operation with respect to the clock frequency at the inner circumference side, where the transmission rate is relatively low, becomes relatively narrow at the outer circumference of the optical disc where the transmission rate is relatively high, resulting that it is difficult to perform the stable pulling-in operation at the outer circumference, which is a serious problem in a practical sense.
On the other hand, the sweep range suitable for the pulling-in operation with respect to the clock frequency at the outer circumference where the transmission rate is relatively high, becomes relatively too wide at the inner circumference side of the optical disc, resulting in that the time required for pulling-in becomes too long at the inner circumference, which is another problem.
Further, in an optical disc reproducing apparatus using this kind of PLL circuit, in case that the CAV reproducing operation is performed with respect to the CLV disc, the clock frequency range to deal with is too wide, so that the problems same as the above may be raised.